With recent progress in nanotechnology, nanoparticles of different materials and sizes have been synthesized and engineered as key components in various applications ranging from solar cell technology to the detection of biomolecules. Meanwhile, nanoparticles generated by vehicles and industry have become recognized as potential threats to health and environment. Microscopy and spectroscopy techniques have played central roles in single nanoparticle/molecule detection. However, their widespread use has been limited by bulky and expensive instrumentation, long processing time, and/or the need for labeling. Light scattering techniques, while suitable for label-free detection, are hindered by the extremely small scattering cross-sections of single nanoparticles.
Interest in nanoparticle detection and characterization techniques has increased with the increasing awareness of the potential benefits and risks of the continuously generated byproduct or massively synthesized nanoparticles. Nanoparticles of special interests range from biological agents and virions to specially synthesized semiconductor, metal, and polymer nanoparticles. Detection and characterization of biological agents and virions is important for biodefense applications and early detection of pandemic outbreaks, while detection and characterization of synthesized nanoparticles is important for a broad range of applications in nanotechnology.
At least some known particle detection systems use conventional microscopic techniques which, despite their high sensitivity and resolution, may not be suitable for field measurements due to their expensive and bulky constructions, long processing times, and the necessity of pretreatment (labeling with fluorescent dyes, etc.) of the particles. Further, at least some known optical particle counters use light scattering measurements to allow field measurements and detect and count individual particles or groups of particles. These counters generally require off-axis detectors for the collection of the scattered light, bulky configurations, and relatively sophisticated signal processing components.
There is a growing interest for nanoparticle detection using nano and micro-scale sensors, which, with relatively high sensitivity, also have the potential for in-situ sensing. Some nano/micro-scale sensors detect particles by monitoring resonance frequency changes caused by additional effective mass of binding particles, while resonator-based micro/nano-optical resonator sensors rely on either resonance frequency shift or mode splitting due to changes in the effective polarizability of the resonator system upon particle binding. Resonator-based sensors have shown to detect and count individual nanoparticles having a radius as small as radius 30 nanometers (nm). This high sensitivity is attributed to the resonance-enhanced interaction between the particle and the evanescent tail of the light field due to tight light confinement and extended interaction time provided by the resonator. These sensors generally require a fiber taper to couple the light into and out of the resonator from a tunable laser, whose wavelength is continuously scanned to monitor the changes in the resonance modes, thus making these highly compact and sensitive sensors relatively expensive.